Process for the cementation of iron and steel



Patented Mar. 10, 1931 UNITED STATES PATENT OFFICE HERMANN FREUDENBERG, OF FRANKFORT-ON-THE-MAIN, GERMANY, ASSIGNOR, BY MESNE ASSIGNMENTS, TO THE ROESSLER & HASSLACHER CHEMICAL COMPANY, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE PROCESS FOR THE CEMENTATION OF IRON AND I STEEL No Drawing.

This invention is concerned with the cementation and hardening of objects made from iron, iron alloys, refined steel, or special alloy steel and the like. I wish objects of the above nature to be included in the term iron articles as used hereinafter and in the appended claims. It is already known that the cementation of such objects can be carried out in a fused bath which contains a salt, such as sodium chloride, soda and the like, which becomes liquid at the temperature used for the hardening operation and which contains a cementing material in the form of a cyanide. The cyanide is decomposed upon dipping the objects to be hardened in the bath, whereupon the carbon and the nitrogen of the same penetrates into the surface of the objects to be hardened.

I have now found that one can carry through the hardening operation with exceptionally good results by using in the melt finely divided carbon as the cementing material.

Salts which may be used in the fused bath of my process may be sodium chloride, barium chloride and other salts which do not give rise to troublesome reactions. Low melting mixtures of salts, as for example, sodium chloride and soda, or potassium chloride and soda or combinations of sodium chloride, potassium chloride and soda, have proved especially satisfactory.

According to the invention, the cementation is carried out in the above mentioned fused bath in the presence of finely divided carbon. 'This carbon may be added as such to the salt mixture constituting the fused bath or it may be formed during the heating process from substances containing carbon, as

for example, milled wood charcoal, peat charcoal, active charcoal, finely divided graphite or the like: The objects to be'treated are brought into the bath in the usual manner and are held therein for some time at the proper carburizing temperature. The quantity of the finely divided charcoal incorporated in the fused bath will usually amount to about one or two percent of the weight of the bath. The cementing temperature will be adjusted according to the depth of cemen- Application filed .Tanuary 12, 1929. Serial No. 332,227.

tation desired. Higher temperatures favor the rapid penetration of carbon. The cementation can be carried out at temperatures of about 800900 C. By the use of higher temperatures, for example, 900950 C. or above, one can obtain cementation to a greater depth in a given periodof time or the period of time usually required for a given carburization can be shortened. Temperatures that are too high are to be avoided, particularly when working over long periods of time, since this may cause the core of the object to be converted into a coarsely crystalline state.

After cementation the articles to be hardened are then quenched in the usual way by ilnlimersion in water, mineral oil, brine or the Example I.A bolt of iron which was poor in carbon was heated to 950 C. for two hours in a fused bath consisting of soda and sodium chloride in which there was incorporated 1% of very finely milled wood charcoal. The bolt was then quenched in the usual manner. The hardness of the bolt was increased by this procedure from 13 on the Rockwell scale to 42 Rockwell, whereas in a fused salt bath of the same composition but'free of carbon, the same kind of a bolt heated to the same temperature and for the same length of time and then quenched, only showed a hardness of 20 Rockwell.

Example II.A bolt of iron Which contained a small amount of carbon was heated at 950 C. for one and a half hours in a bath of potossium chloride and sodium chloride in which about 2% of milled active carbon was suspended. After quenching, the bolt showed a hardness of 58 Rockwell, while the same kind of a bolt treated in the same manner in the same kind of a bath, with the exception that it contained no free carbon, showed only 20 Rockwell.

Furthermore I have found that I can add with advantage to the salt bath compounds such as the hydroxides or cyanides of alkali metals, as for example, sodium hydroxide, potassium hydroxide, sodium cyanide, and thereby facilitate considerably the penetration of the carbon into the iron surface. The addition of such substances to the melt comprising finely divided carbon shortens the duration of the immersion process when it is intended to obtain a certain depth of penetration of carbon or it results in a'deeper penetration into the surface when the article is left in the fused bath for the same length of time as, in a bath without the additional substances such as sodium hydroxide or the like.

These substances may be used in mixture with other salts providing they are liquid at the carburizing temperature and do not give occasion to troublesome side reactions. Particularly suited for assisting the process is sodium hydroxide.

The quantity of these compounds added may vary within .wide limits. In some cases a few per cent are suflicient for considerably reducing the time necessary for cementation. Thus I have found that I was able to reduce the cementation time of iron articles by immersing "them in a fused bath containing finely milled charcoal by the addition of 5 to 10 percent of sodium hydroxide or, for instance, 12 to 15% sodium cyanide, to less than half the time which is required for obtaining the same depth of peneration when working without the addition of the foregoing compounds at the same temperature. The accelerating action of these additions may be combined with a carburizing action in those cases, where the additional substance has a carburizing effect of its own, as for example, sodium cyanide, by increasing the quantity added to the fused bath.

Example [l[.-A bolt of iron low in carbon is immersed in a fused bath of parts sodium chloride, 50 arts potassium chloride containing 3 parts nely milled wood charcoal to which 5 to 10 parts sodium hydroxide are added. After an immersion of two hours at 950 centrigrade, the depth of carbon penetration is one millimeter.

A similar bolt immersed in a bath without sodium hydroxide, all other conditions being equal, was carburized to 0.1 millimeter only.

Ewample IV.An iron bolt with a carbon content of 0.06% C. is left in a bath containing equal parts of potassium chloride and sodium chloride, in which 3% finely milled charcoal is suspended. After two hours at 850 C. the depth of carbon is less than 0.1 mm. After adding 12% NaCN to the fused bath a bolt treated in exactly the same way shows a penetration depth of 1 millimeter.

In the place of sodium hydroxide or' sodium cyanide alone, mixtures of more than one of these substances may be used.

In order to prevent the melt becoming too vpoor in carbon or charcoal respectively the latter substances may be added continuously to the fused bath advantageously in small portions. The addition of charcoal or active carbon oflers great difiiculties as the carbon mamas has the tendency of rising to the top of the melt. On trying to incor orate the fine coal powder into the fused ath .as, by stir ring, the latter starts foaming and may run over. l have found that I can overcome this difliculty by first fusing one or more components of the bath in mixture with, forinstance, wood charcoal, and then adding this mixture either in the warm liquid state or as a solidified mass. When added in this form the fine charcoal powder is easily distributed throughout the whole bath.

Moreover, when auxiliary substances such as sodium hydroxide are used for carrying out my invention the latter compounds have also to be replaced as they, too, are used up and converted into sodium carbonate. In this case I prefer to fuse the additional carbon, such as woodcharcoal, with the sodium hydroxide and fill up the required quantities of both substances by addition of this mixture. The proportion of, for instance, wood charcoal and alkali metal hydroxides such as caustic soda to be fused together I adapt to the special requirements of the bath i. e. to the ratio in which the charcoal and the hydroxide respectively have been used up.

In general I have found that a ratio of about one kilogram caustic soda toabout 250 to 300 grms. wood charcoal fulfils these requirements. 1 gradually add such quantlties of this mixture to a melt containing sodium chloride and potassium chloride as will main tain a concentration of charcoal of approximately 1 to 3% and of caustic alkali metal of 5 to 10%.

What I claim is:

1. In the process of hardening iron articles by immersing the article in a fused salt bath and then quenching the step which comprises adding finely divided carbon to the bath and enhancing the carburizing efiect by addition of alkali metal hydroxide.

2. In the process of hardening iron articles by immersing the article in a fused salt bath and then quenching the step which comprises adding finely divided carbon to the bath and enhancing the carburizing 'efi'ect by addition of sodium hydroxide.

3. In the process of hardening iron articles by immersing the article in a fused salt bath and then quenching, the step which comprises adding about 1 to 3 parts by weight of finely divided carbon to 100 parts of a non-carburizing alkali metal chloride bath and enhancing the carburizing effect by the addition of about 4 times as much alkali metal hydroxide as the amount of carbon added.

4:. In the process of hardening iron articles by immersing the article in a fused salt bath and then quenching the step which comprises adding about 1 to 3 parts by weight of finely divided carbon to 100 parts of a non-carburizing alkali chloride bath and enhancing the carburizing effect by addition of about a times as much sodium hydroxide as the amount of carbon added.

5. In the process of hardening iron articles by immersing the article in afused salt bath and then quenching the step which comprises adding finely divlded carbon to the bath, augmenting the carburizing effect by addition of alkali metal hydroxide and supplementing the carbon by introducing the latter in mixture with one or more components of the bath after this mixture has been fused previously.

Signed at Frankfort-on-the-Main, Germany, this 22nd day of December. A. D. 1928.

HERMANN FREUDENBERG. 

